Autoplay: add Feature flag to turn on unified autoplay policy.

Bug: 715049
Change-Id: I5fd2e635fe686ad50bb1bc22ae2a1ca0b64a4246
Reviewed-on: https://chromium-review.googlesource.com/738098
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511856}
2 files changed